1. Field of the Invention
This invention relates to a fuel cell power generating system and more particularly to a fuel cell power generating system in which the power generating efficiency is improved.
2. Description of the Prior Art
FIG. 1 is a diagrammatic illustration showing a prior art power generating system using a molten carbonate fuel cell which is disclosed, for example, in GRI Report No. FCR-3522-2 (1981), entitled "Evaluation of Natural Gas Molten Carbonate Power Plant". In FIG. 1, the power generating system illustrated includes a molten carbonate fuel cell 1 composed of singular or plural cell stacks and having a fuel gas electrode and an oxidant gas electrode, a temperature and humidity regulating means 2 for regulating the temperature and the humidity of the fuel gas exhausted from the molten carbonate fuel cell 1, a combustor 3 for oxidizing the unreacted components in the fuel gas exhausted from the molten carbonate fuel cell 1, a heat exchanger 4 for carrying out the excess heat, which is generated in the molten carbonate fuel cell 1, to the exterior of the system, and a gas circulation means for circulating a part of the exhaust gas discharged from the fuel cell 1 as a heat transfer medium for cooling the fuel cell 1.
In a fuel processing device such as a reforming device, a coal gasification device or the like, the fuel gas, obtained by reforming hydrocarbons, alcohols, coals or the like, contains, as its principal components, hydrogen, carbon monoxide and carbon dioxide. In order to avoid deposition of the carbon owing to the decomposition of the carbon monoxide under a high temperature condition (for example, over 400.degree. C.), as shown in the following formula (1), the fuel gas, being added by an appropriate amount of water vapor, is supplied to the fuel gas electrode of the molten carbonate fuel cell 1. EQU 2CO.revreaction.C.dwnarw.+CO.sub.2 ( 1)
The water vapor contained in the fuel gas would consume the carbon monoxide according to the formula (2), so that the deposition of the carbon can be prevented. EQU CO+H.sub.2 .revreaction.CO.sub.2 +H.sub.2 O (2)
Excess moisture in the fuel gas exhausted from the fuel gas electrode of the molten carbonate fuel cell 1 is removed as drain by the temperature and humidity regulating means 2. After adjustment of the temperature and moisture of the fuel gas, unreacted combustible materials contained therein are oxidized throughly by the combustor 3, and then supplied to the oxidant gas electrode of the molten carbonate fuel cell 1. On the other hand, air, supplied from air supplying means (not shown) such as a blower, is mixed with the fully oxidized combustion gas supplied from the combustor 3, and then fed to the oxidant gas electrode of the molten carbonate fuel cell 1 as an oxidant gas.
The molten carbonate fuel cell 1 operates at around 650.degree. C., and the following electrochemical and chemical reactions (3) and (4) will occur at the fuel gas electrode and the oxidant gas electrode, respectively, of the fuel cell 1. (at the fuel gas electrode) EQU H.sub.2 +CO.sub.3.sup.2- .revreaction.H.sub.2 O+CO.sub.2 +2e.sup.-( 3) EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 ( 4)
(at the oxidant gas electrode) EQU 1/2 O.sub.2 +CO.sub.2 +2e.sup.- .revreaction.CO.sub.3.sup.2-( 5)
As a result of these reactions, the molten carbonate fuel cell 1 converts the chemical energy of the fuel gas to electrical energy and heat energy. The heat energy thus produced is removed from the molten carbonate fuel cell 1 by the circulating oxidant gas which is circulated as a heat transfer medium through the oxidant gas electrode and the heat exchanger 4 by the gas circulation means 5. The removed heat is transmitted to the heat exchanger 4 and is discharged therefrom to the exterior of the system.
The power generating efficiency of the fuel cell system is influenced by various factors. Among them, important factors are a fuel utilization ratio and an average single cell voltage. These two factors are mutually correlated so that when the fuel utilizing ratio is too much increased, the average single cell voltage will be decreased considerably because of dilution of the reactant materials such as hydrogen or carbon monoxide at the outlet of the fuel cell 1. This tendency appears significantly in the molten carbonate type fuel cell in which the reaction products, for example, water vapor or carbon dioxide, are diffused to the fuel gas side, as shown in formula (3). FIG. 2 shows one example of the correlation between the fuel utilization ratio and the average single cell voltage in the molten carbonate fuel cell 1.
In the fuel cell power generating system, the most effective measure for improving the power generating efficiency is to increase the fuel utilization ratio. In the prior art power generating system, however, if the fuel utilization ratio increases, the average single cell voltage decreases extremely. Accordingly, effective improvement in the power generating efficiency has not been accomplished.
On the other hand, when the fuel utilization ratio increases, the partial pressure of the water vapor contained in the fuel gas increases at the outlet of the fuel cell. This results in the problems that the corrosion of various metallic members are accelerated and the decomposition of the electrolyte is promoted as shown in the formula (6), particularly in the case of the molten carbonate fuel cell. Therefore, the lifetime of the fuel cell will be shortened. EQU LiKCO.sub.3 +H.sub.2 O.revreaction.LiK (OH).sub.2 +CO.sub.2( 6)
Another type of a prior art power generating system is known which is diagrammatically illustrated in FIG. 3. This power generating system includes, in addition to the same components 1 through 5 as those shown in FIG. 1, a second molten carbonate fuel cell 6 composed of singular or plural cell stacks and having a fuel gas electrode and an oxidant gas electrode, the second fuel cell 6 being disposed between the first molten carbonate fuel cell 1 and the temperature and humidity regulating means 2; and a temperature adjusting means 7 which is interposed between the first and second molten carbonate fuel cells 1 and 6 for adjusting the temperature of the fuel gas fed from a fresh fuel gas and a fresh water vapor, which are the same as those fed to the fuel gas electrode of the first fuel cell 1 and are admixed with the exhaust fuel gas discharged from the first fuel cell 1. The fuel gas exhausted from the electrode of the second fuel cell 6 is fed to the temperature and humidity regulating means 2, and the oxidant gas electrodes and of the first and second fuel cells 1 and 6 are supplied with the fully oxidized gas from the combustor and air. Also, a part of exhaust gases discharged from the oxidant gas electrodes and of the first and second fuel cells 1 and 6 is circulated through the heat exhanger 4 and the oxidant gas electrodes and by means of the gas circulation means 5.
The above-described fuel cell power generating system illustrated in FIG. 3 operates substantially in the same manner as the first-mentioned system shown in FIG. 1 does, except for the following. Specifically, the fuel gas or exhaust gas, exhausted from the fuel gas electrode side of the first molten carbonate fuel cell 1 and containing unreacted combustible components and water vapor, is admixed with a fresh fuel gas and a fresh water vapor, of which compositions are the same as those
In the second molten carbonate fuel cell 6, the same reactions as those in the first molten carbonate fuel cell 1 take place for generating electric power while preventing carbon deposition. The fuel gas exhausted from the second molten carbonate fuel cell 6 is introduced to the temperature and humidity regulating device 2 wherein the surplus water vapor in the exhaust gas is condensed and removed, and the temperature and the humidity of the fuel gas are appropriately adjusted. Then, the unreacted combustible materials in the exhaust gas discharged from the fuel gas electrode side of the second fuel cell 6 are completely oxidized by air in the combustor 3 and supplied to the first and second molten carbonate fuel cells 1 and 6. The heat taken out by the heat exchanger 4 from the exhaust gas discharged from the oxidant gas electrodes of the first and second fuel cells 1 and 6 is used, for example, for producing water vapor so as to run a steam turbine for generation of electric power as a bottoming cycle. Also, water vapor to be mixed with the fuel gas can be produced by utilizing the taken-out heat.
With the second-mentioned fuel cell power generating system, however, a large quantity of water vapor must be added to the fuel gas for the purpose of avoiding carbon deposition so that the amount of water vapor, utilizable for the bottoming cycle, for example, is necessarily reduced. Accordingly, the power generating efficiency of this system as a whole is low.